1. Field of the Invention
The invention relates to a method for casting a molten metal cast piece provided with at least one through-opening. The cast pieces referred to here are typically cylinder crankcases for high-capacity combustion engines which are cast from a cast iron metal.
2. Description of Related Art
Modern combustion engines are constantly being developed in order to reduce fuel consumption. Reducing the volume and weight of the components is key here. This trend is described among experts as ‘downsizing’. The aim of ‘downsizing’ is, for example, to achieve performances with smaller engine sizes that previously required a larger overall installed size.
Successful downsizing of combustion engines requires inter alia enhancement of the technological properties of their individual components. Thus, the achievable performance can be more than trebled with modern engine designs at the same installation size.
Cast iron with vermicular graphite is sometimes used today instead of conventional cast iron to ensure the adequate resilience of cast iron cylinder crankcases at said power density, or high alloy cast iron materials are used to achieve the required strength.
The cast pieces of the type described above are typically cast in casting moulds, which are made up of several moulded parts and casting cores. Whilst moulded parts generally determine the external shape of a cast piece, casting cores are placed into casting moulds to represent recesses, cavities, through-openings and similar in the cast piece to be produced.
Depending on their position in or on the cast piece and the ease with which they can be removed from the mould after the cast piece has set, moulded parts and casting cores are configured as permanent moulded parts and permanent casting cores or as lost moulded parts and casting cores. Whilst permanent moulded parts and casting cores consist of materials, which can withstand the stresses and strains that occur during casting, and therefore can be used repeatedly for a large number of casting processes, lost moulded parts and casting cores usually consist of moulding materials which can be destroyed easily by the application of force or the effect of temperature. If a casting mould consists, entirely, or at least to a substantial extent, of lost moulded parts and casting cores, it is usually referred to as a lost mould, whereas casting moulds, which consist primarily of permanent, moulded parts, are referred to as permanent casting moulds even if lost casting cores are placed therein. Lost moulds are typically used for cast iron casting, whilst, permanent, casting moulds or a combination of permanent moulded parts and lost moulded parts are frequently used in light metal casting.
Lost moulded parts and casting cores are typically made of moulding materials consisting of sand mixed with an appropriate binder, which hardens when producing the respective moulded parts or casting cores as a result of a chemical reaction, provided it retains adequate dimensional stability until the molten mass cast in the casting mould sets. The components of the moulding material can be coordinated here such that the respective casting core or moulded part automatically breaks into pieces while the casting piece is cooling as a result of the stresses and strains that occur. Alternatively, or additionally, the disintegration of lost moulded parts and casting cores can be effected by applying mechanical forces. Thus, for example, casting cores can be destroyed by shaking the respective cast piece into such tiny pieces that the moulding material thereof automatically trickles out of the cast piece, or the destruction of the casting cores is speeded up by drilling, extrusion or flushing. The prerequisite for this, however, is that the cast piece is substantially completely cool so that the stresses and strains occurring during the mechanical or thermal destruction of the lost casting cores and moulded parts does not result in damage to the cast piece.
The process of cooling the cast piece has a crucial influence of its mechanical properties. Problems may occur when cooling a cast piece in that the cast piece cools at different rates in different areas as a result of uneven distribution of material or an irregular heat supply. Internal stresses and strains may occur in the cast piece as a result of such uneven cooling, which may lead to a dramatic deterioration of its mechanical loading capacity.
In order to minimise the occurrence of such stresses and strains, cooling from the casting temperature to a temperature usually below 600° C. is performed deliberately slowly when casting cast pieces with wall thicknesses that vary considerably. The casting plants used in practice are equipped with cooling sections of a specific length for this purpose, wherein said cooling sections may also include ‘cooling stations’ where the casting moulds containing the cast pieces to be cooled can dwell for a specific period in order to further delay cooling. If no means are available to guarantee sufficiently slow cooling, or if internal stresses and strains that are too high are still present in the cast piece even after such slow cooling, the cast pieces must be subjected to additional annealing in order to reduce the respective stresses and strains.
As an alternative option for minimising the tensile stresses in the inner region of a cylinder crankcase, DE 10 2008 048 761 A1 suggests cooling the molten metal after it has been poured into the casting mould in a directed manner such that setting of the molten mass is effected firstly inside the cast piece or a region of the cast, piece directed towards a feeder head is set. It should be possible to achieve this by influencing the setting of the respective cast piece by means of different cooling capacity of at least two independent cooling circuits provided on the respective casting mould. However, this can only be accomplished if the respective casting mould is configured as a permanent casting mould at least in the regions in which the cooling capacity is intended to be applied in a targeted manner. Specially formed sleeves are thus provided for moulding the cylinder openings of the respective cylinder crankcase, which are drawn out of the casting without damage after setting. It has proven advantageous for the removal of the sleeves after setting, if cooling of the edge of the cylinder openings is started at a different time from the cooling of the cylinder surface and the cylinder edge is cooled at a different intensity from the cooling of the cylinder surface. In this manner, the setting of the cast cylinder crankcase in the region of the cylinder openings can be performed such that the cylinder crankcase can be removed from the mould at a point when although it is set, it is still at a high temperature.
Another option for targeted accelerated cooling of cast piece regions, which are arranged inside the respective component part, is described in DE 11 2006 000 627 T5. The sand casting mould known from this document for producing a cast piece made of an aluminium alloy comprises a portion, which is formed by means of a solvent, more particularly water, soluble binder, and a further portion which is formed by means of a binder, which cannot be dissolved using the respective solvent. This division of the sand mould portions enables removal of the core formed on the basis of the soluble binder by applying pressure with the solvent, i.e. by applying pressure by means of a jet of water, for example, and consequently the inner regions of the cast piece exposed to the effect of the solvent cool more rapidly that the rest of the cast piece. Said solution only applies to cavities, which are present in the cast piece, and requires a complex design of the sand mould from different moulding materials.
Another suggestion for accelerated cooling of the regions of a cast piece surrounding a through-opening, designed for a special application scenario and suitable for light metal casting, is made in DE 10 2010 003 346 A1. In the method described here for casting a piston for a combustion engine, once the surface layers in the region of the piston pin bores nave set, the sleeves provided for removing said bores from the mould are drawn back and the region of the respective bore is cooled by means of a cooling agent, which is supplied through at least one of the sleeves.
Against the background of the prior art described above, the problem to be solved by the invention consisted in providing a method, which makes it possible to produce cast pieces with through-openings having optimum mechanical properties in a manner that requires minimal outlay in terms of equipment.